The following relates to the nuclear reactor arts and related arts.
In nuclear reactor designs of the pressurized water reactor (PWR) type, a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel. In a typical design, the primary coolant is maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. Primary coolant flows upward through the reactor core where it is heated and rises through the central riser, discharges from the top of the central riser, and reverses direction to flow downward back toward the reactor core through a downcomer annulus defined between the pressure vessel and the central riser. The nuclear core is built up from multiple fuel assemblies each comprising a bundle of fuel rods containing fissile material (usually 235U).
In a typical PWR design, upper internals located above the reactor core include control rod assemblies with neutron-absorbing control rods that are inserted into/raised out of the reactor core by control rod drive mechanisms (CRDMs). Conventionally, the CRDMs employ motors mounted on tubular pressure boundary extensions extending above the pressure vessel. In this design, the complex motor stator can be outside the pressure boundary and magnetically coupled with the motor rotor disposed inside the tubular pressure boundary extension. The pressure vessel of the PWR is conventionally connected with an external pressurizer and external steam generators via large-diameter piping.
More recent small modular reactor (SMR) designs have been driven by a desire to make the PWR more compact and to have fewer large diameter vessel penetrations. Toward this end, in so-called “integral” PWR design, the steam generator is located inside the pressure vessel, typically in the downcomer annulus. This replaces the external primary coolant loop carrying radioactive primary coolant by a secondary coolant/steam loop carrying nonradioactive secondary coolant. The use of an internal pressurizer comprising a steam bubble at the top of the pressure vessel and suitable baffling is contemplated to eliminate the large-diameter penetration for the external pressurizer. Still further, fully internal CRDM motors are contemplated, which eliminates the tubular pressure boundary extensions above the reactor vessel. Some illustrative PWR designs incorporating these advances are described in, e.g.: Thome et al., “Integral Helical-Coil Pressurized Water Nuclear Reactor”, U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which is incorporated by reference in its entirety; Malloy et al., U.S. Pub. No. 2012/0076254 A1 published Mar. 29, 2012 which is incorporated by reference in its entirety; Stambaugh et al., U.S. Pub. No. 2010/0316177 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety; and DeSantis, U.S. Pub. No. 2011/0222640 A1 published Sep. 15, 2011 which is incorporated herein by reference in its entirety.
Such SMR designs introduce numerous challenges not faced in more conventional PWR designs. One such challenge is reactor refueling. SMR components are tightly packed into a compact pressure vessel, with the nuclear reactor core located at or near the bottom of the pressure vessel in order to minimize the possibility of the primary coolant water level falling to a point that exposes the reactor core during a loss of coolant accident (LOCA). This means that all major components are located above the reactor core, and must be removed in order to access the reactor core for refueling. For example, in PWR designs disclosed in Stambaugh et al., U.S. Pub. No. 2010/0316177 A1, an upper internals basket welded to a mid-flange of the pressure vessel supports the internal CRDMs and the control rod guide frames, and power and signal connections for the CRDMs are routed to connectors on the mid-flange. These components must be removed in order to access the reactor core.
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